Unmanned aerial vehicles (UAVs) require range and durability in the air. To help achieve this, engines and their control systems need to be relatively light in weight and yet provide high performance.
One way to achieve lightness in weight is to omit as many non-essential components and control systems as possible and/or make such components lightweight. However, to survive in the extreme ambient conditions that UAVs are exposed to, the engine and its control systems also need to be robust and reliable.
The ignition system is one essential engine control system. Without an ignition system the UAV engine will not start or continue to run.
Minimising components in the ignition system, particularly where this may impact on any redundancy capability, can however lead to an increased risk of engine failure, and ultimately potentially complete loss of the UAV.
UAVs need to be reliable because the consequences of mechanical or electrical failure (such as engine mechanical failure or engine sub-system failure e.g. the ignition system) can be very severe. If a UAV in some way fails whilst flying, it comes down to earth very quickly, often with catastrophic results in terms of the UAV crashing. Also, such failure can happen when the UAV is a long way from its base and potentially over dangerous or difficult terrain, making safe recovery risky or impossible. There is also the associated risk of losing very expensive surveillance or other equipment carried by the UAV.
The need for reliability becomes particularly crucial if UAVs are used in civilian areas where a failure could endanger human lives.
To maintain reliability, additional and/or more robust components can be used. However, this tends to lead directly to increased UAV weight. Robustness of components tends to require more material or features in order for the components to better withstand shock, vibration, extremes of temperature and changes in temperature. This need for robustness of components is all the more relevant for UAVs which have to cope with harsh environmental conditions experienced at high altitude for long periods of time.
Furthermore, in terms of reliability, having only one ignition system on-board does not allow for any failure at all of the ignition system. This may not satisfy certain regulatory requirements for such aerial vehicles when used in specific applications.
It would hence be beneficial to have full redundancy in the ignition system. This could obviously be achieved by duplicating the main ignition system. Duplication is readily achieved by having a secondary ignition system exactly replicating or mirroring the components and capability of the main ignition system.
A benefit of such duplication of the main ignition system is that components are exactly the same between the two systems, making spare parts and maintenance easier to manage.
Replicating the main ignition system with an entire ignition system of the same components will also enable full performance of the UAV to be maintained in order to complete a flight without returning to point of origin.
However, merely duplicating the primary ignition system can significantly impact desired weight (and often size) constraints of the UAV which can drastically reduce performance and range of the UAV.
A known system employing duplicated ignition systems is disclosed in U.S. Pat. No. 6,357,427. Multiple controllers are employed, each controller controlling operation of a spark plug and able to control the energy and timing of two spark plugs in the event of failure of one of the controllers. As suggested by the background section of U.S. Pat. No. 6,357,427, this system is aimed at relatively large, commercial aircraft with low speed, large cylinder engines (for example, 1.5 liters per cylinder) where the large spark plug gap at cold engine temperature start-up requires a higher energy than operating the engine at running temperature. U.S. Pat. No. 6,357,427 seeks to optimise ignition control, but in terms of redundancy in the ignition system, U.S. Pat. No. 6,357,427 seeks only to exactly replicate the components and capability of the controller and spark plug arrangement. Each controller operates exactly the same as another, with each spark plug receiving the same timing control signals and outputting the same energy as any other spark plug for given engine conditions. If one controller fails, the other controller acts as a back-up, providing the same functionality and having exactly the same capability as the failed controller.
Another known system where ignition system components are merely replicated is disclosed in US 2006/0235601. Disclosed is an aviation ignition system which is completely replicated in a back-up ignition system. Crank speed and cam timing sensors are replicated, as are individual CPUs, coils and spark plugs across primary and back-up ignition systems. The primary and back-up CPUs continuously ‘talk’ to each other, sharing ignition data, to ensure that if one system fails, the other exactly matches the ignition spark timing and energy of the other to ensure no change in engine operation. As with U.S. Pat. No. 6,357,427, directly replicating all of the components from the primary ignition system in the back-up ignition system adds considerable weight, cost and complexity that the present invention seeks to alleviate.
The aircraft engines and aircraft themselves, to which the systems of U.S. Pat. No. 6,357,427 and US 2006/0235601 are applicable, are of the type where the added weight, cost and complexity of exactly duplicated systems is of little or no concern. The capability of the secondary/back-up ignition system in these solutions exactly duplicates that of the primary ignition system. There has been no consideration in either solution of the need to reduce weight, complexity or functionality of the back-up ignition system.
For certain inductive coil ignition systems, the inductive coil has a heavy iron core which is required so that the coil can produce a spark with sufficient energy to initiate combustion. Adding a second inductive coil to provide redundancy capability for the ignition system of a UAV would hence add significant weight to the UAV. This is particularly true of spark ignition UAV engines running on heavy fuels, such as JP5 and JP8 where a robust coil is required to generate sufficient spark energy to initiate combustion of such heavy fuels.
Also, duplicating the primary ignition system significantly invariably adds to the cost of the UAV ignition system, not least because the primary ignition system components needing to be robust are therefore relatively expensive.
Whilst it would be beneficial to have full ignition system redundancy with two matching (duplicated) ignition systems for a heavy fuel UAV engine, the added weight of the complete second system would be too detrimental to performance and the effect on cost would also be an issue. An ignition system along these lines which includes two standard ignition systems, one serving as a primary system, and one for backup/redundancy purposes, would make for an uncompetitive overall engine package.
With the aforementioned in mind, it is desirable for the present invention to provide an ignition system for a UAV that provides ignition system redundancy capability but alleviates the problem of adding significantly to the weight (and cost) of the UAV.
The known means of achieving redundancy involves direct duplication of parts, as is required by civil aviation authorities for many aircraft systems. However, duplication of parts is in conflict with the requirement to keep weight to a minimum. Accordingly, a “partially” redundant ignition system is proposed, which fulfils the need for operational redundancy without incurring a significant weight (and cost) penalty incurred by such direct duplication.